The invention relates to a high pressure swivel comprising an inner annular wall and an outer annular wall concentric with said inner wall and rotatable relative to said inner wall. Each annular wall has an outer surface and an inner surface and comprises an aperture extending in a radial direction from the inner surface towards the outer surface. The apertures in the walls are in mutual fluid communication, the inner surfaces of the walls facing each other in close proximity and defining an annular gap extending in an axial direction on each side of the apertures. At least one flexible sealing element is comprised in the annular gap on each side of the aperture in each annular wall. The apertures are respectively in fluid communication with a fluid inlet or is a fluid outlet.
Such high pressure swivels are known from EP-B-0013454 and EP-B-0013455. For the purpose of the present invention the term xe2x80x9chigh pressurexe2x80x9d is intended to include pressures of about 50 bar and higher. In the known high pressure swivels, a toroidal chamber is defined between the inner, stationary annular wall, or ring, of the swivel and the outer annular wall or ring. In the following description the terms xe2x80x9cannular wallsxe2x80x9d and xe2x80x9cringsxe2x80x9d will be used in an interchangeable manner. Conduits which convey high pressure hydrocarbons such as oil or gas at pressures of between 100 and 300 bar, extend through the central space of the inner, stationary ring. Through the inlets of the inner ring, the conduits connect to the toroidal chamber. The rotating outer ring that defines one half of the toroidal chamber, comprising one or more outlets that are connected to said toroidal chamber. On either side of the toroidal chamber of the known swivels, the sealing elements are located in the annular gap, also called the xe2x80x9cseal extrusion gapxe2x80x9d.
The sealing of swivels handling liquids or crude oils with limited gas content is achieved with a double sealing arrangement on either side of the toroidal chamber. The so-called primary seals are positioned closest to the toroidal chamber and carry the full pressure, while the secondary seals are positioned behind the primary seals to back-up the primary sealing function in an event of primary seal rupture.
The sealing of gas or crude with high gas content is more difficult due to the low viscosity of the gas and the danger of fire and explosions in case of leakage. Therefore, for such an application a different sealing concept is used. The primary seals are replaced by two pairs of seals facing each other. Within these two pairs of seals, a barrier of liquid is maintained at a higher pressure than the gas to be sealed, such that an absolute gas tightness is achieved. Leakages of seals in this configuration will only result in internal leakages of the barrier liquid to the gas line or to a leak recuperation tank.
Defects in the known swivels occur when the width of the annular gap in which the seals are located exceeds a certain level such that the seals are extruded from the annular gap by the high pressure acting upon them. The design of the known swivels has concentrated on minimizing the seal extrusion gap opening. On the other hand, a minimum gap width must be maintained such that deflections of the inner and outer rings of the swivel will not cause the two parts to contact one another and do thus prevent rotation. In order to minimize the deformations of the inner and outer ring due to pressure effects, the known rings are made of cast steel, with a wall thickness of about 240 mm for a 2 m diameter swivel. Deformations are prevented by the heavy masses of the inner and outer rings. A single swivel of a seal diameter of 1980 milimeters suitable for operating at pressures of 100 bar may weigh 20 tons or more.
Not only should the deformations of the inner and outer rings be minimized, but known swivels are also designed in such a way that stresses remain within the limits which are imposed by the Pressure Vessel Codes. The known swivel units are often stacked one on top of the other. The hydrocarbons supplied to the swivels often have temperatures between 20 and 120xc2x0 C. During transfer of these fluids, the outer ring of the swivel is cooled by the external ambient air, whereas the inner ring remains relatively hot. This is especially so when the swivel is exposed to outside weather conditions and when a wind blows against a swivel stack. The temperature difference between the concentric rings of the swivel generates deformations of the extrusion gap as the inner ring expands more than the outer ring. To avoid contact between the rotating parts of the swivel due to temperature fluctuations, when a pressure drop occurs, the initial extrusion gap should be wide enough to accommodate these temperature deformations. The width of the initial extrusion gap that is required to accommodate the thermal deformations reduces the allowable deformations due to pressurization.
In order to minimize the variations in the extrusion gap dimensions due to temperature effects, the outer ring of the swivel is often insulated by a suitable insulating material such as rubber. These insulations often cause problems as the design of the swivel stack becomes more complicated and inspection of the swivel unit becomes difficult.
Finally, to further increase the stiffness of the inner and outer rings in the areas of the inlets and outlets, reinforcements are attached to the inner and outer rings around the inlets and outlets of the known swivels. These reinforcements again add to the weight of the swivel.
It is therefore an object of the present invention to provide a high pressure swivel which is relatively insensitive to pressure variations and which can transfer high pressure fluids while using a relatively light weight construction.
It is a further object of the present invention to provide a swivel for transferring high pressure fluids, in which temperature fluctuations of the inner and outer ring are reduced.
It is a further object of the present invention to provide a swivel which can be combined with other swivels of similar shape in a number of different configurations.
It is again an object of the present invention to provide a swivel which is of a relatively small size, which is of a simple construction, which is relatively maintenance free and which can be easily manufactured at relatively low cost.
Thereto the swivel according to the present invention is characterized in that the apertures in the inner and the outer annular walls are connected to the fluid inlet or the fluid outlet via a respective first and second distribution member, the apertures having a smaller axial dimension than the diameter of the fluid inlet and/or the fluid outlet.
Because the axial dimension of each aperture in the inner and outer ring is significantly reduced compared to the dimensions of the fluid inlet and the fluid outlet, the pressure area between the sealing elements, on which the forces act that tend to separate the inner and the outer rings, is also significantly decreased in size. In this way a very small size and light weight swivel can be constructed. The invention is based on the principle of connecting two relatively big pressure vessels, formed by the inlet and the outlet, via the distribution members, to a rotatable connecting element (inner and outer annular walls) of reduced pressure area. Thereby, the forces on the connecting element remain limited and this element can consequently be constructed with reduced dimensions. The distribution members may be formed by a plurality of individual conduits, by distribution chambers or by a combination thereof.
In one embodiment, the apertures have a smaller axial dimension than the axial dimension of the distribution members. In this way, fluids are passed to the relatively large size distribution members for distribution of the fluids towards or away from the apertures along the circumference of the inner and outer rings.
In another embodiment of a swivel according to the present invention, each annular wall comprises a plurality of apertures distributed along its circumference, each aperture having a smaller throughflow area than the fluid inlet and the fluid outlet. Because the plurality of apertures around the circumference of the inner and outer annular walls of the swivel according to the present invention, the througflow areas of the inlet and outlet are distributed, via the distribution members, over N smaller subareas, for N apertures.
The inner and outer rings of the swivel according to the present invention may comprise a central toroidal chamber which can be of rectangular, oval or circular cross-section. It is however also possible for the inner and outer ring to comprise a single slot or a number of slots extending along the circumference of each ring, the slots of the inner and outer ring being distributed in such a way that always a number of slots of the inner and outer ring are overlapping for any relative rotational position of the inner and outer ring. In a further embodiment the parts of annular walls and of the first and second distribution members that in an axial direction are located between the sealing elements and in a radial direction are located between the outer surfaces of the annular walls, are substantially symmetric with respect to the annular gap. Due to the substantially symmetric arrangement of the swivel around the pressure area, instabilities caused by the pressure fluctuations are prevented, and the swivel is made especially suitable for use at pressures above 50 bar.
The distribution members according to the present invention can be formed by a number of pipes connected to each aperture in the inner and outer ring. The distribution members are preferably concentric with the inner and outer rings, but need not be. It is for instance possible that the inlet and the outlet are connected to the apertures in the inner and outer rings of the swivel by means of a plurality of flexible or rigid conduits. In one embodiment of the swivel according to the present invention, each distribution member comprises a distribution chamber and a number of distribution ducts. Each distribution duct is on one side connected to a respective opening in the rings and is on its other side connected to a distribution chamber. The distribution chambers are connected to the fluid inlet or the fluid outlet and have a throughflow area which corresponds to the throughflow area of the fluid inlet or the fluid outlet to which the respective chamber is connected. Preferably the distribution chambers are toroidal, one chamber being disposed within the inner annular wall and the other chamber being disposed around the outer annular wall of the swivel. In the above construction, deformations that are caused by the fluid pressure in the concentric distribution chambers, are less than for the inner and outer annular walls of the swivel, and hence help to minimize the seal extrusion gap.
Furthermore, by the concentric configuration a heat barrier is placed around each annular wall of the swivel, such that temperature fluctuations of the annular walls are minimized.
In addition, due to the larger number of relatively small apertures in the inner and outer annular walls of the swivel, the structure remains relatively rigid. The weakening effects of the relatively large size throughflow areas of the inlets and the outlets are minimized because the inlets and outlets are located at a relatively large radial distance from the annular gap carrying the seals. Hence deformations that normally would occur due to the large sized opening in the annular walls of known swivels, are minimized.
Finally, the construction according to the present invention is to a large extent symmetrical with respect to the extrusion gap, such that it is self balanced and distortions due to pressure are minimized thereby.
It should be noted that a two-fluid swivel for low pressure applications is known from the article xe2x80x9cSingle Point Mooringxe2x80x9d, by Roger Maari, 1985, printed in Monaco, page 182, ISBN 2-9500554-0-0). In this known construction, hydrocarbons are distributed from an inlet duct to a central chamber located within the inner annular ring and from there, through apertures in the inner annular wall. Via apertures in the outer annular wall, the fluids reach an outer ring-shaped distribution chamber which connects to the fluid outlet. The inner surfaces of the annular walls of the known low pressure are not in close proximity, such that a narrow annular gap (seal extrusion gap) between these surfaces is not formed. Furthermore, the known construction is asymmetric in the areas near the openings in the inner and outer annular wall. The construction is therefore unsuitable for high pressure purposes. Moreover, the known structure is relatively complex and of a large size. The known swivel can not be combined into a vertical or concentric assembly.
A highly preferred embodiment of the swivel according to the present invention comprises two distribution chambers concentric with the annular walls. A first distribution chamber is disposed radially inwardly from the inner annular wall and a second distribution chamber: is disposed radially outwardly from the outer annular wall. Each distribution chamber is enclosed by an annular section of the outer surface of the respective annular wall and by an enclosing wall. This enclosing wall is with a first and second end connected to the outer surface of the respective annular wall in a fluid tight manner, axially outwardly from the apertures in the annular wall. In this way, the distributions chambers are directly attached to each annular wall. It is preferred that the distribution chambers are placed along the majority of the height of the annular rings of the swivel. Temperature variations of the rings are best prevented by distribution chambers extending along substantially the height of the inner and outer rings. Furthermore, distortions of each annular chamber according to this preferred embodiment are transferred to the annular walls of the swivel in such a way that the extrusion gap at the position of the seals is narrowed. Hence a self-closing effect takes place. It is preferred that the distribution chambers are substantially of circular cross-section, such that distortions under pressure are minimized.
Finally, It has appeared that the distribution chambers according to the present invention reduce deposit of sand and debris on the seals. Most of the debris will be caught in the first distribution chamber connected to the inlet. Hereby the seal life can be extended.
The swivel concept according to the present invention allows a thin-walled construction which can be produced using curved pipe sections and plates that are welded together. Thereby heavy castings can be avoided such that a cheap and light weight swivel is obtained.
Due to its substantially reduced weight, the swivel assembly according to the invention can not only be easily stacked in a vertical assembly, but can also be placed in a concentric configuration. For comparison, a known swivel having fluid inlets and outlets with a 12 inch diameter a seal diameter of 1980 millimeters and a maximum operating pressure at 100 bars of a standard xe2x80x9cpiston sealxe2x80x9d design and made according to conventional casting technology, has a weight of approximately 20 tons and a maximum extrusion gap opening under pressure of 0,35 millimeters. A swivel having same characteristics and made according to the present invention has a weight of 7 tons and having a maximum extrusion gap at the primary seals of 0.14 millimeters. This small extrusion gap allows a pressure increase by a factor of 2 compared to the known swivel construction at a third of the weight of the known construction.